Certain monopolistically competitive industries exist with
important entry barriers. In such industries firms are able to maintain profits over the
long run. In other words, owners earn more than they would in
their next best alternative activity.

In most such industries the entry barrier is a license of
some type, one that must be purchased or earned through education
and testing. If a license is easy to obtain it isn't much of a
barrier to entry and so profits will be nearly zero.